1. Field of the Invention
The present invention relates to communication apparatuses that perform wireless communication operations while avoiding interference with existing communication systems and coexisting with the communication systems. More particularly, the present invention relates to a communication apparatus capable of flexibly selecting the frequency that is used by acquiring the surrounding radio environment with its radio device and adoptively switching the communication parameter of the radio device in accordance with the acquired surrounding radio environment.
2. Description of the Related Art
Prevalence of wireless communication technologies in recent years is accompanied by a problem of the shortage of frequency resources that has become obvious. Solutions to such problems include new exploitation of millimeter waves and effective use of micro waves that have been used. As a method for the effective use of the micro waves, a “cognitive radio” technology attracts attention. The cognitive radio technology adaptively performs optimal communication in order to avoid interference with existing communication systems and to coexist with the communication systems.
In the cognitive radio communication, a communication system or a radio communication apparatus temporarily shares the frequency band that is originally allocated to another communication system or another radio communication apparatus with the other communication system or the other radio communication apparatus on the assumption that interference with the other communication system or the other radio communication apparatus is avoided. Specifically, the cognitive radio communication is realized by a radio device, such as a terminal or a base station, which acquires the surrounding radio environment and adoptively switches the communication parameter of the radio device in accordance with the acquired surrounding radio environment to flexibly select the frequency that is used. Since the cognitive radio technology is capable of actively achieving the effective use of frequencies, the cognitive radio technology is much expected in recent years as a communication technology that greatly and fundamentally changes the way how to ensure the radio system in related art staring from the allocation of the frequencies. For example, standardization of the cognitive radio communication is promoted in IEEE802.22.
It is necessary to periodically check whether the frequency band is used because the radio communication channels to be used may not be constantly available in the cognitive radio communication. Accordingly, the cognitive radio communication has a problem in that the ratio of the check time to the communication time is increased. For example, a radio communication apparatus is proposed, in which the check is performed in a first time period, the data communication is performed in a second time period, the sum of which and the first time period is equal to a constant value, and the first and second time periods are determined in accordance with the frequency band selected through the check to realize the efficient cognitive radio communication (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-60410).
With the view of the effective use of frequencies, the relationship between an existing radio system and a cognitive radio system is of importance in order to share the same frequency band between the existing radio system and the cognitive radio system. Selectively using the frequency band of one radio device in an existing communication system from multiple frequency bands that are prepared as candidates for the communication bands in accordance with the surrounding radio environment is called Dynamic Spectrum Access (DSA). When an existing radio system is referred to as a “primary system” and a cognitive radio system is referred to as a “secondary system”, it is necessary to minimize the level at which the secondary system interferes with the primary system (hereinafter referred to as interference from the secondary system to the primary system) and the level at which the primary system interferes with the secondary system (hereinafter referred to as interference from the primary system to the secondary system) in the DSA (refer to FIG. 4).
Of methods for sharing frequencies in the DSA, a method receives the most attention in which the primary system does not perform interference management and the secondary system autonomously determines the communication parameter so as not to affect the primary system for the communication.
Detect and Avoid (DAA) applied an ultra-wideband (UWB) radio system using a lower band is familiar as a technology for reducing the interfering level from the secondary system to the primary system. For example, in a multi-band Orthogonal Frequency Division Multiplexing (OFDM) communication system, adopting a low-interference (Low-IF) method at the receiver side prevents a dead band from occurring near the local frequency of the receiver and adopting a Zero-IF method at the transmitter side prevents an image spurious signal caused by the difference in amplitude and phase between IQ signals in quadrature modulation from being generated. This eliminates the need to concurrently perform the avoidance in the DAA both in units of subbands and in units of subcarriers, thereby reducing the load of a countermeasure against the DAA in a baseband unit (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-258904).
A radio communication apparatus is proposed, in which the level of an analog signal that is received is detected to perform the determination by the DAA before the analog signal is subjected to digital conversion, a digital circuit having a function of selecting the frequency is driven when an interfering wave from another communication system is detected to determine the narrowband signal, which is the interfering wave, in units of subcarriers by the DAA, and a notch is made only in the frequency band having the interfering wave in a transmission system to perform the UWB communication. In this radio communication apparatus, it is possible to detect an interfering wave with a smaller consumption power and to avoid the interference in consideration of the efficiency of frequency usage in the UWB communication adopting the OFDM modulation method (for example, refer to Japanese Unexamined Patent Application Publication No 2007-166068).
However, the DAA is highly specialized for UWB radio system and it may be difficult to accommodate the DAA to general cognitive radio systems in a broader sense.